the important publishing centres. They remained so until the later 19th century. They required the illustrations to be printed separately, on a different type of printing press, so encouraging illustrations that took a whole page, which became the norm. Engraving and etching gave sharper definition and finer detail to the illustrations, and rapidly became dominant by the late 15th century, often with the two techniques mixed together in a single plate. A wide range of books were now illustrated, initially mostly on a few pages, but with the number of illustrations gradually rising over the period, and tending to use more etching than engraving. Particular kinds of books such as scientific and technical works, children's books, and atlases now became very heavily illustrated, and from the mid-18th century many of the new form of the novel had a small number of illustrations. Luxury books on geographical topics and natural history, and some children's books, had printed illustrations which were then coloured by hand, but in Europe none of the experimental techniques for true colour printing became widely used before the mid-19th century, when several different techniques became successful. In East Asia colour printing with many different woodblocks was increasing widely used; the fully developed technique in Japan was called nishiki-e, and used in books as well as ukiyo-e prints. Lithography (invented by Alois Senefelder in 1798 and made public in 1818) allowed for more textual variety and accuracy. This is because the artist could now draw directly on the printing plate itself. New techniques developed in the nineteenth and twentieth centuries revolutionized book illustrations and put new resources at the disposal of artists and designers. In the early nineteenth century, the photogravure process allowed for photographs to be reproduced in books. In this process, light-sensitive gelatin was used to transfer the image to a metal plate, which would then be etched. Another process, chromolithography, which was developed in France in the mid-nineteenth century, permitted color printing. The process was extremely labor-intensive and expensive though as the artist would have to prepare a separate plate for each color used. In the late twentieth century, the process known as offset lithography made color printing cheaper and less-time consuming for the artist. The process used a chemical process to transfer a photographic negative to a rubber surface before printing. There were various artistic movements and their proponents in the nineteenth and twentieth centuries